linux-kernel - Re: [PATCH v2] cpuidle: Use nanoseconds as the unit of time

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Message-ID: <20191108093919.GJ4114@hirez.programming.kicks-ass.net>
Date:   Fri, 8 Nov 2019 10:39:19 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Doug Smythies <dsmythies@...us.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH v2] cpuidle: Use nanoseconds as the unit of time

On Fri, Nov 08, 2019 at 02:44:13AM +0100, Rafael J. Wysocki wrote:
> On Thu, Nov 7, 2019 at 3:25 PM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > Currently, the cpuidle subsystem uses microseconds as the unit of
> > time which (among other things) causes the idle loop to incur some
> > integer division overhead for no clear benefit.
> >
> > In order to allow cpuidle to measure time in nanoseconds, add two
> > additional fields, exit_latency_ns and target_residency_ns, to
> > represent the exit latency and target residency of an idle state
> > in nanoseconds, respectively, to struct cpuidle_state_usage and
> > initialize them with the help of the corresponding values in
> > microseconds provided by drivers.  In addition to that, change
> > cpuidle_governor_latency_req() to return the idle state exit
> > latency constraint in nanoseconds.
> >
> > With that, meeasure idle state residency (last_residency_ns in
> > struct cpuidle_device and time_ns in struct cpuidle_driver) in
> > nanoseconds and update the cpuidle core and governors accordingly.
> >
> > However, the menu governor still computes typical intervals in
> > microseconds to avoid integer overflows.
> 
> Since this addresses all of the comments received by the RFC version
> that was posted over a month ago and I don't see any more issues with
> it, I'm tempted to simply queue it up for 5.5 unless somebody sees a
> good enough reason why that would be a bad idea.

Nah, have at.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>